Meal plans created by the American Diabetes Association (ADA) provide about 45% of calories from carbs. This includes 45–60 grams per meal and 10–25 grams per snack, totaling about 135–230 grams of carbs per day.
However, a growing number of experts believe people with diabetes should be eating far fewer carbs than this. In fact, many recommend fewer carbs per day than what the ADA allows per meal.
This article takes a look at the research supporting low-carb diets for diabetics and provides guidance for determining optimal carb intake.
What Are Diabetes And Prediabetes?
Glucose, or blood sugar, is the main source of fuel for your body’s cells.
In people with diabetes, the body’s ability to process and use blood sugar is impaired.
Although there are several types of diabetes, the two most common forms are type 1 and type 2 diabetes.
Type 1 Diabetes
In type 1 diabetes, the pancreas is unable to produce insulin, a hormone that allows sugar from the bloodstream to enter the body’s cells. Instead, insulin must be injected to ensure that sugar enters cells.
Type 1 diabetes develops because of an autoimmune process in which the body attacks its own insulin-producing cells, which are called beta cells. This disease is usually diagnosed in children, but it can start at any age, even in late adulthood.
Type 2 Diabetes
Type 2 diabetes is more common, accounting for about 90% of people with diabetes. Like type 1 diabetes, it can develop in both adults and children. However, it isn’t as common in children and typically occurs in people who are overweight or obese.
In this form of the disease, either the pancreas does not produce enough insulin or the body’s cells are resistant to insulin’s effects. Therefore, too much sugar stays in the bloodstream.
Over time, the beta cells of the pancreas can wear out as a result of pumping out more and more insulin in an attempt to lower blood sugar. They can also become damaged from high levels of sugar in the blood.
Diabetes can be diagnosed by an elevated fasting blood sugar level or an elevated HbA1c level, which reflects blood sugar control over a period of two to three months.
Diabetes is diagnosed by a fasting blood sugar of at least 126 mg/dL (7 mmol/L) or an HbA1c of at least 6.5%.
Before type 2 diabetes occurs, blood sugar levels are elevated but not high enough to be diagnosed as diabetes. This stage is known as prediabetes.
Prediabetes is diagnosed by a blood sugar level between 100–125 mg/dL (5.6–6.9 mmol/L) or an HbA1c between 5.7–6.4%.
While not everyone with prediabetes goes on to develop type 2 diabetes, it’s estimated that approximately 70% will eventually become diabetic.
What’s more, even if prediabetes never progresses to diabetes, people with this condition may still be at an increased risk of heart disease, kidney disease and other complications related to high blood sugar levels.
Bottom Line: Type 1 diabetes is due to the destruction of pancreatic beta cells, while type 2 diabetes occurs when cells are resistant to insulin or the pancreas doesn’t make enough. Prediabetes often progresses to diabetes.
How Does Food Affect Blood Sugar Levels?
Blood sugar levels are affected by many things, including exercise, stress and illness.
However, one of the major factors affecting your blood sugar is the type and amount of food you consume.
Of the three macronutrients — carbs, protein and fat — carbs have the greatest effect on blood sugar by far.
This is because when your body digests carbs, they are broken down into sugar, which enters your bloodstream.
This occurs with all carb-containing foods, such as refined carb sources like chips and cookies, as well as healthy types like fruits and vegetables.
However, whole foods contain fiber. Unlike starch and sugar, naturally occurring fiber in food does not raise blood sugar levels.
When diabetics eat foods high in digestible carbs, their blood sugar can rise to very high levels. Eating large portions of these foods typically requires high doses of insulin or diabetes medication to control blood sugar.
Because they are unable to produce insulin, people with type 1 diabetes need to inject insulin several times a day, regardless of what they eat. However, eating fewer carbs can significantly reduce their mealtime insulin dosage.
Bottom Line: Your body breaks down carbs into sugar, which enters the bloodstream. People with diabetes who eat a lot of carbs require insulin or medication to keep their blood sugar from rising too much.
Research On Carb Restriction For Diabetes
There are many studies supporting the use of carb restriction in diabetics.
Very Low-Carb, Ketogenic Diets
Very low-carb diets typically induce mild to moderate ketosis, a state in which your body uses ketones and fat, rather than sugar, as its main energy sources.
Ketosis usually occurs at daily intakes of less than 50 grams of total carbs, or 30 grams of digestible carbs (total carbs minus fiber). This would be no more than 10% of calories on a 2,000-calorie diet.
Very low-carb, ketogenic diets have been around for many years. Indeed, doctors prescribed them for people with diabetes before insulin was discovered in 1921.
Several studies have shown that restricting carb intake to 20–50 grams of carbs per day can significantly reduce blood sugar levels, promote weight loss and improve heart health markers in people with diabetes.
In addition, these improvements often occur very quickly.
For instance, in a study of obese people with diabetes, limiting carbs to 21 grams per day led to a spontaneous decrease in calorie intake, lower blood sugar levels and a 75% increase in insulin sensitivity in two weeks.
In a small, three-month study, people were randomized to consume a calorie-restricted, low-fat diet or a low-carb diet containing up to 50 grams of carbs per day.
The low-carb group had an average decrease in HbA1c of 0.6% and lost over twice as much weight as the low-fat group. Also, 44% of the low-carb group discontinued at least one diabetes medication, compared to 11% of the low-fat group.
In fact, in several studies, insulin and other diabetes medications have been reduced or discontinued due to improvements in blood sugar control.
Diets containing 20–50 grams of carbs have also been shown to lower blood sugar levels and reduce the risk of disease in people with prediabetes.
In a small, 12-week study of obese, prediabetic men who followed a Mediterranean diet limited to 30 grams of carbs per day, fasting blood sugar dropped to 90 mg/dL (5 mmol/L), on average, which is well within the normal range.
In addition, the men lost an impressive 32 pounds (14.5 kg) and experienced significant reductions in triglycerides, cholesterol and blood pressure, among other beneficial health effects.
Importantly, these men no longer met the criteria for metabolic syndrome due to reductions in blood sugar, weight and other health markers.
Although concerns have been raised that higher protein intake on low-carb diets may lead to kidney problems, a recent 12-month study found that very low carb intake did not increase the risk of kidney disease.
Bottom Line: Many studies have shown restricting carbs to 20–50 grams, or less than 10% of calories, per day may lower blood sugar, promote weight loss and improve health markers in people with diabetes and prediabetes.
There is currently no agreement on how many carbs a low-carb diet contains.
However, for purposes of this article, the term “low-carb” will refer to diets containing 50–100 grams of carbs, or 10–20% of calories, per day.
Although there are very few studies on carb restriction for type 1 diabetics, most that exist have used carb intakes in the low-carb range. In each case, impressive results have been reported.
In a long-term study of people with type 1 diabetes who were advised to restrict carbs to 70 grams per day, those who complied reduced their A1c from 7.7% to 6.4%, on average. What’s more, their A1c levels remained the same four years later.
A 1.3% reduction in HbA1c is a significant change to maintain over several years, particularly in those with type 1 diabetes.
One of the biggest concerns for people with type 1 diabetes is hypoglycemia, or blood sugar that drops to dangerously low levels.
In a 12-month study, adults with type 1 diabetes who restricted daily carb intake to less than 90 grams had 82% fewer episodes of low blood sugar than before they started the diet.
People with type 2 diabetes may also benefit from limiting their daily carb intake to 50–100 grams, or up to 20% of calories.
In a small, five-week study, men with type 2 diabetes who consumed a high-protein, high-fiber diet providing 20% of calories from carbs per day experienced a 29% reduction in fasting blood sugar, on average.
Bottom Line: Research suggests a daily carb intake of 50–100 grams, or 10–20% of calories, may lower blood sugar levels in people with diabetes. It may also help reduce episodes of low blood sugar in people who take insulin.
Moderate, Low-Carb Diets
As with “low-carb” diets, there is no formal definition for “moderate, low-carb diets.” In this article, “moderate low-carb” will refer to diets providing 100–150 grams of digestible carbs, or 20–35% of calories, per day.
A few studies looking at diets within this range have reported good results in people with diabetes.
In a 12-month study of 259 people with type 2 diabetes, those who followed a Mediterranean diet providing 35% or fewer calories from carbs experienced a significant reduction in HbA1c, from 8.3% to 6.3%, on average.
Bottom Line: A few studies have found that diets providing 100–150 grams of carbs, or 20–35% of calories, per day may lower blood sugar levels in people with diabetes.
How Low Should You Go?
Research has confirmed that many levels of carb restriction effectively lower blood sugar levels.
Since carbs raise blood sugar, reducing them to any extent can help control how much your blood sugar rises after eating.
For instance, if you’re currently consuming about 250 grams of carbs daily, reducing your intake to 130 grams should result in significantly lower blood sugar after meals, as the few, moderate, low-carb studies that are available have shown.
However, most studies have looked at blood sugar response in people with diabetes who consume 20–50 grams of carbs per day.
This level appears to produce the most dramatic results, including blood sugar improvement that reduces or even eliminates the need for insulin or diabetes medication.
Bottom Line: Studies have shown that a daily carb intake between 20–150 grams, or 5–35% of calories, may effectively manage diabetes and prediabetes. Very low-carb diets may have the greatest effect.
High-Carb Foods To Limit Or Avoid
There are many tasty, nutritious, low-carb foods that do not raise blood sugar levels very much.
These can be enjoyed in moderate to liberal amounts on low-carb diets.
However, there are certain foods that should be limited or avoided because of their high carb content:
- Breads, muffins, rolls and bagels
- Pasta, rice, corn and other grains
- Potatoes, sweet potatoes/yams and taro
- Legumes like peas, beans and lentils (except green beans, snow peas and peanuts)
- Milk and sweetened yogurt
- Most fruit, except for berries
- Cake, cookies, pie, ice cream and other sweets
- Snack foods like pretzels, chips and popcorn
- Juice, soda, sweetened iced tea and other sugar-sweetened drinks
Keep in mind that not all of these foods are unhealthy. For example, fruits and legumes can be highly nutritious. However, they can be problematic for diabetics who are trying to manage their blood sugar levels by eating fewer carbs.
On the other hand, you can eat plenty of low-carb vegetables, nuts, seeds, avocados, meat, fish, eggs, full-fat dairy and other foods.
Bottom Line: Although many delicious foods can be included on a low-carb diet, those that are high in carbs should be limited or avoided altogether.
Are Low-Carb Diets Always Best For Diabetes?
Low-carb diets have consistently been shown to lower blood sugar and improve other health markers in people with diabetes.
However, certain higher-carb diets have also been credited with similar beneficial health effects.
For example, some studies on low-fat vegan and vegetarian diets suggest that this way of eating may lead to better blood sugar control and overall health.
In a 12-week study, a brown rice-based vegan diet containing 268 grams of total carbs (72% of calories) lowered people’s HbA1c more than the standard Korean Diabetes Association diet containing 249 grams of total carbs (64% of calories).
An analysis of four studies found that people with type 2 diabetes who followed a low-fat, macrobiotic diet containing about 70% of calories from carbs achieved significant reductions in blood sugar and other health markers.
The Mediterranean diet has also been shown to improve blood sugar control and provide other health benefits in individuals with diabetes.
However, it’s important to note that these diets weren’t directly compared to low-carb diets, but rather to standard, low-fat diets often used for diabetes management.
In addition, there aren’t as many studies on these diets as there are on low-carb diets. While they may be effective for certain people, more research is needed to confirm these findings.
Bottom Line: Studies suggest that certain higher-carb diets may be effective for diabetes management. However, research comparing these diets to low-carb diets is needed.
How To Determine Optimal Carb Intake
Although studies have shown that many different levels of carb intake may help control blood sugar, the optimal amount varies by individual.
It is worth repeating that diets containing 20–50 grams of carbs per day have been studied the most and typically produce the most dramatic results in diabetics.
However, in addition to keeping blood sugar levels within a healthy range, it’s important to eat the amount of carbs at which you feel best, as well as that you can realistically maintain in the long term.
Therefore, figuring out how many carbs to eat requires some testing and evaluating to find out what works best for you.
To determine your ideal carb intake, measure your blood sugar with a blood glucose meter before a meal and again one to two hours after eating.
The maximum level your blood sugar should reach is 139 mg/dL (8 mmol/L) in order to prevent damage to blood vessels and nerves.
However, you may want to aim for an even lower upper limit.
To achieve your blood sugar goals, you may need to restrict your carb intake to less than 10 grams, 15 grams or 25 grams per meal.
Also, you may find that your blood sugar rises more at certain times of the day, so your upper carb limit may be lower for breakfast than lunch or dinner.
In general, the fewer carbs you consume, the less your blood sugar will rise and the less diabetes medication or insulin you’ll require to stay within a healthy range.
If you take insulin or diabetes medication, it’s very important to speak with your doctor or health care provider prior to reducing your carb intake so that your dosage can be adjusted to prevent low blood sugar.
Bottom Line: Determining the optimal carb intake for diabetes management requires testing your blood sugar and making adjustments as needed based on your response, including how you feel.
Take Home Message
Based on the evidence to date, conventional recommendations that diabetics should consume at least 45% of their daily calories from carbs appear misguided.
Multiple studies have shown that a daily carb intake of 20–150 grams, or between 5–35% of calories, not only leads to better blood sugar control but may also promote weight loss and other health improvements.
Therefore, a carb-restricted approach may be your best bet to keep your blood sugar within a healthy range.
However, even among people with diabetes, some individuals can tolerate more carbs than others.
Testing your blood sugar and paying attention to how you feel at different carb intakes can help you find your own personal range for optimal diabetes control, energy levels and quality of life.